Due to characteristics of the unique spatial structure as well as special functions, the automobile steering knuckle workpiece generally needs the left workpiece and right workpiece to be used in pairs; however, since the left workpiece and right workpiece are different in structure, two sets of fixtures are needed under normal circumstances in machining to respectively machine the left and right workpieces, thus not only increasing the machining cost, but also bringing about adverse impact on the machining efficiency. Even when the left workpiece and right workpiece are fixed on one fixture to be simultaneously machined, it is very inconvenient to install and dismount the workpieces as the limitation of the workpiece structure and the working space exists; the present invention provides a combination fixture, which can satisfy the simultaneous machining of the left and right workpieces of the steering knuckle and realize the convenience in clamping and dismounting.